The Rail Prototype Wind Tunnel Data and Calculations

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Our primary objective in sending 9 wheels to the wind tunnel was to see where the prototype of the Rail stacked up against our existing wheel offerings as well as the Zipp 404 Firecrest clincher. The challenge when you are sifting through the pretty substantial amount of data generated at the tunnel is how to present it in a way that it will be both meaningful and accurate. So we've developed a couple of charts which give you the executive summary, which we'll fill in with some context. 

The raw data is provided in grams of drag and aero watts, neither of which tell the complete story about how a wheel really performs. More commonly, you'll see wheels in the 40K TT standard, which shows how much time each theoretically saves over a benchmark wheel in a 40K TT at 30mph. That's what we show here:

Usually the benchmark used is a 32 spoke Mavic Open Pro, which is a ridiculous proxy for the "average" wheel on a TT bike but which makes it easy for a brand to claim they saved a big enough number of seconds to raise eyebrows. We compared everything to our FSW 23 wheelset, which is 21mm high and 23mm wide with 20 raidally-laced Sapim CX-Ray spokes. If you're concerned enough about aerodynamic performance to look at wind tunnel data, the FSW is much more likely to approximate the wheel you are currently using, or the one you would buy if you decide not to go with a deep carbon wheel. 

You will see from the data that under these test criteria, the Rail is faster than any of our current wheels, including the RFSC 85s which are 33mm deeper. It is also only 2 seconds slower than the Zipp 404 FC, despite having 8 more spokes in the prototype and being 6mm shallower. Our internal goal for deciding whether to proceed with the Rail was that it test faster than our current RFSC 58s and within a couple aero watts (about 6 seconds in this test) of the RFSC 85s. When comparing to the 404, we are not limiting the Rail to aerodynamics, as the improved road feel of the larger inside width is really the heart of the Rail's story. We also wanted to bring the depth down to 52mm to broaden its use range. So if we were within about 3-4 watts (9-12 seconds) of the Zipp at this stage we would have been satisfied. Coming in at faster than our 85s and within 2 seconds of the Zipp made our decision easy; we've already greenlighted the Rail for production.

The 40K TT at 30mph comparison is a standard, but an unfortunate one for consumers trying to fairly evaluate how different wheels would perform for them. Aerodynamics are amplified at higher speeds (as Dave points out here, particularly int he comments), but the average speed for most TTs and bike legs in triathlons is well under that. Wheel brands like the standard though because it shows a larger gap against benchmark wheels. To get a better sense of how each wheel performs at mortal speeds, have a look at the following chart:

The slower you are going, the less delta you see in the aero drag between each wheelset. If you want to do the math and figure out the 40K TT time savings at different speeds, use these calculations:

30mph: 11 grams of drag = 1 aero watt
25mph: 9 grams of drag = 1 aero watt
20mph: 7.5 grams of drag = 1 aero watt 

If 1 aero watt equals a difference 3 seconds in a 40K TT, the 39 second advantage of the 404s over the FSW alloys at 30mph drops down to 25 seconds at 25mph and 13 seconds at 20mph. 

Some people will ask about Angles of Attack (or you might ask about yaw in which case you mean Angles of Attack, as per Dave), knowing that they vary at different speeds. The faster you riding, the more likely you are to encounter lower angles of attack. At 30mph, over 80% of AOAs are at 10 degrees or below. At 20mph the sweet spot is between 10-20 degrees, comprising about 50% of all real-world conditions. The data above already reflects the distribution of AOAs at the different speeds.

We'll roll out more of our test data and findings over the next few days. You can sign up to receive future posts by email here if you don't want to keep hitting the refresh button. Until then, fire away with any questions in the comments. 


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  • PYF on

    Thanks Mike for your answer.It seems I was not patient enough… Looking forward to reading the next articles then !Thanks again !

  • Mike May on

    PYF, we'll be sharing more wind tunnel data this week, including the curves at different AOAs as well as a chart or two showing the impact of the 4 additional spokes that we tested. It ends up being a ton of data and charts altogether so we spaced it out to allow for some conversation on each element separately.

  • PYF on

    Sorry … one more question, what was the influence of the 4 spokes on the 2 RFSC 58mm you brought, one with 20 spokes, one with 24 spokes ?It's great that you tested that as we often wonder how much we loose when we go from the lighter more aero option to a safer and stiffer option with more spokes…Actually we can measure how much weight we add / save, we can measure how much stiffness we gain / loose but the aero part is always an unknown.As you say 4 spokes aero difference on the RFSC 58 is not going to translate to the same aero difference on another wheel, but it gives an idea of how much it "can" matter, just like the great information you offered on spoke shape ;-) !

  • PYF on

    Hi,Awsome article, thanks for sharing those results !If I may ask, in your previous aero articles (summer TDF articles) you showed the aero graphs with grams of drag in "Y" coordinate and angle in "X" coordinate. It would be great to see the same graphs with your A2 results to see where does The Rail stall as opposed to the Zipp Firecrest but also to your more traditionnal shape actual rims.Think you could add this information to the article ?Thanks a lot and keep up the great work, awsome products you are working on and awsome information you share with us !

  • Cedrick G. on

    Fantastic news guys, congrats on validating the design, that in and of itself is a great achievement. Can't wait to hear about timeline, but take the time and make them awesome.More wishlist stufff while we're on the subject – Low Sideways Deflection? (can't remember the term). I've been on a ton of wheels that rode great – transferred power well, cornered well etc – only to have that completely ruined when you slam them against the pads in a sprint. Esp with new wide format wheels. I know you can loosen your brake, but sounds like the rail is going to be very wide and it just sucks generally to be rolling with a soft brake.Good, but not loose radius for mounting and unmounting tires. Some wheels just have poor tolerances that can making getting tires on and off really annoying, I'm sure you've got this already. keep up the great work!Ced



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